Novel synthetic chemistry and formulations are continuously being developed to improve the optical performance and longevity of commercial and consumer-grade paints and coverings. Formulations are tailored to accommodate a variety of environmental factors and the substrates onto which the paints are being applied.
The paints employed by fine artists are generally made using traditional formulations employing historical manufacturing practices. Today there is a strong interest amongst artists for paints that are compositionally similar to the paints employed by the historical art masters, but where they also exhibit improved long term color stability.
In many cases the pigments or dyes employed in fine arts paints and pastels are highly susceptible to color degradation over time. Environmental exposure, UV light degradation originating from sunlight and room light, atmospheric oxidation, and reaction with other compounds present in the painted substrate are all common drivers that result in color fading. In the interest of artwork longevity, modern artists are forced to avoid traditional paints or pastels that are known to fade or degrade over time. Manufacturers of art supplies have also avoided pigments or dyes that, while desirable for their color brilliance or other optical properties, are known to be difficult to work into paints through chemical reactivity, resistance to uniform dispersion, toxicity, or undesirable changes in color hue or tone after the paint has dried or cured.
Classical paints also commonly employ silica particles as bulk filler or in some cases as a means to enhance the paint luster or transparency. We present an improved method for protecting pigments or dyes where the protecting layer is silica, and where the pigment or dye provides all of the desirable color and other optical properties the fine art painter desires.
For the purpose of this disclosure the terms ‘artist color media’ relates to colored materials applied to a solid surface by artists in the production of artwork. Artist color media may include paints, pastels, colored pencils, chalks, stains, glazes, and clays. Example solid surfaces may be essentially flat or contoured and may include canvas, paper, wood, glass and stone. The term ‘traditional artist color media’ relates to color media with compositions used for more than about 100 years.
Color and Colorants
‘Color’ is the general term which applies to the entire subject of visible light. Any given color can be described in terms of its value and hue. ‘Hue’ is the term for the pure spectrum colors commonly referred to by the ‘color names’—red, orange, yellow, blue, green violet—which appear in the rainbow. ‘Value’ is defined as the relative lightness or darkness of a color.
‘Pigments’ are inorganic or organic, colored, white or black materials which are essentially insoluble in the medium in which they are incorporated. ‘Dyes’, unlike pigments, do dissolve during their application and in the process lose their particulate structure. It is thus by physical characteristics rather than by chemical composition that pigments are most often differentiated from dyes.
For the purpose of this disclosure the term ‘colorant’ relates to any free pigments or dyes, regardless of their solubility, which are used to create a color in the silica particle. ‘Pigment’ relates to colorants that are insoluble, and ‘dyes’ relate to colorants that are soluble in the desired solvent.
The term ‘colored particle’ refers to the silica particle substrate which contains at least one colorant, without any silica encapsulation layer. The term ‘pigment particle’ refers to the final silica encapsulated colored particle.
Encapsulation of Pigments
The presence of the encapsulating silica in accordance with the invention helps to passivate the surface properties and functionality of the core colored particle, thereby enabling good pigment particle dispersability in a variety of paint and pastel compositions. Pure silica surfaces are preferable to surfaces containing other metal species due to the increased reactive nature of metal species. Encapsulating silica layers are therefore preferably greater than 90% wt/wt silica, and less than 0.1% wt/wt with respect to metal species. Additionally, the surface deposited silica layer is preferably of nanoscale thickness, and optically transparent.
The core-shell type silica-encapsulated colorants in accordance with the invention provide a benefit that the composition of classic paint or pastel compositions can be standardized for such encapsulated pigments without requiring a customized reformulation for a particular pigment type, which is most often required. The silica encapsulation of colorants can therefore enable one to have a different artist color media by merely changing the core colorant composition (pigment) but not the surface shell composition.
Silica Layer Formation Processes
There exist various processes by which the silica shell can be provided on pigment particles. Most of these methods involve a sol-gel reaction. In this process the silica precursor is reacted in a series of hydrolysis and polymerization reactions to form a colloidal suspension, or sol, which is then precipitated onto the pigment core to provide a thin film coating of silica on the core pigment particle. See U.S. Pat. No. 6,113,682 which discloses methods for preparing bulk silica via use of aqueous sodium silicate and pH adjustment, known generally in the art as the sol-gel process.
Another sol-gel system commonly employed to produce silica coatings is based on the hydrolysis of an organosilane precursor reagent such as tetraethoxysilane (TEOS) or tetramethoxysilane (TMOS) in the presence of either water or an alcohol solvent, followed by condensation polymerization of the silicic acid intermediate under basic or acidic conditions (often referred to as a Stober-type method, first published by Stober, Fink and Bohn in J. Colloid Interface Sci. 1968, v. 26, p. 62 the text of which is incorporated herein by reference).
Sol-gel processes are difficult to control and are unable to yield silica particles with narrow distributions of particle sizes or film thicknesses. As a process for creating uniform nanoscale layers of silica on pigment particles, sol-gel processes require tight control of all chemical precursors and careful metering means of acid/base reagents, which in turn results in complex processes which are vulnerable to small variability. As compared to bulk quartz for example, silica coatings created from organosilane precursors such as TEOS and TMOS yields lower density silica matrices as a result of liberation of ethanol or methanol, respectively, when the silica layer is formed. Drying the silica layer serves to remove the liquid phase from the condensed gel, yielding an amorphous, nano-porous glass or micro-crystalline ceramic. Subsequent thermal treatment (firing or sintering) must be performed in order to favor further polycondensation, reduce porosity in the layer, and enhance mechanical properties. In the absence of thermal post-processing, TEOS and TMOS-based silica layers have comparatively poor mechanical strength and can crush or delaminate when subsequent pressure or abrasion is applied to the pigment particles.
Core Silica-Containing Particles
It well known to those skilled in the art that particle size (diameter) can contribute to the degree of optical transparency of the bulk media containing the particles. This behavior relates to the size of the particle relative to the wavelength of the illuminating light. As the particle size approaches the wavelength of the illuminating light the transparency of the bulk media increases. In some applications (for example, cosmetics), transparency is an optical property to be controlled. This control is in part possible by controlling the particle size. Pigment particle sizes suitable for artist color media preferably ranges from 1 to 100 μm, in diameter, and more preferably 5 to 50 μm in diameter. The shape of the particle has also been demonstrated to contribute to the optical properties of the pigment.
With respect to manufacturability of paints and pastels, uniformity and standardization of the pigment particles is desirable. Employing silica particles as substrates that standardize the shape and size of the pigment particles allows for simplification of paint and pastel formulations. Employing silica particles with a specific size and porosity provides a simple means to control the colorant load, and therefore the color intensity and hue formulations may also be standardized.
Core silica containing particles provide a substrate with which to apply colorants, where the result is a colored particle having uniform or well controlled size and shape, as well as colorant load. For the purpose of this disclosure the terms ‘core particles’ and ‘core silica-containing particles’ represent colored pigment particles where the center substrate is composed of silica or silica containing compositions.
For the purpose of this disclosure core silica-containing particles may be spherical or irregularly shaped. Core particles may include other elements beyond silicon, oxygen, and hydrogen; however core particles having a high (SiO2) composition ensure effective bonding to the deposited silica encapsulating layer of the invention. Preferably, in accordance with the invention, the SiO2 composition of the core particle is at least 50% by weight. More preferably, the (SiO2) groups in the core particles preferentially reside on the outer surface of the particle where they are available for additional silica deposition. Core particles may be essentially nonporous or porous. The amount of colorant loading is dependent on the available surface area, and pore volumes (if any) of the core particle, where bulk colorant may accumulate.
Porous Silica Containing Particles: Spherical Porous ‘HPLC’ Silica
Examples of porous silica particles in this disclosure are predominantly spherical, amorphous, and appear white in color in the absence of any chemical treatment. Such particles are commercially available from a variety of suppliers and are commonly used as stationary phase substrates in high pressure liquid chromatography (HPLC) applications. HPLC particles are manufactured with tightly controlled silica purity, particle size, porosity, and surface area. For this application it is recognized that equally suitable porous silica particles may have relaxed tolerances with respect to HPLC-grade silica particles, however control of these physical properties in general can result in greater consistency with respect to the optical and color properties of the finished pigment particle.
In the case of porous silica particles, the pores are filled with colorant materials and the particles assume the color of the colorant. The degree to which the silica particle can assume the color (described here as the hue or value) is largely based on (1) the concentration of the colorant in or on the particle, (2) the surface area of the particle, and (3) the pore volume of the particle.
Nonporous Silica Containing Particles: Talc
Talc is a layered hydrous magnesium silicate of general empirical formula Mg3Si4O10(OH)2, that is a naturally occurring mineral and usually ground to a fine powder. The talc employed in this disclosure is bright white, although naturally occurring colored examples are commercially available. Talc has a high resistance to acids, alkalies and heat. The hydroxy groups normally are internal to the magnesium layer and are not accessible to water except at the edges of the silicate sheet. Thus, conventional talc powder is a hydrophobic material that easily blends and disperses with organic media including polymers but is not easily dispersed in aqueous solvents.
The composition of talc is approximately 64% SiO2. The crystalline structure of talc consists of magnesium sandwiched between silica sheets, rendering the outermost surface predominantly Si—O units which are readily available for additional silica deposition. Talc is essentially nonporous and the surface area of talc increases with decreasing particle size in general agreement with the surface area vs. particle diameter. (see Foster, J; Doll, J. “Particle Size Effect on Talc Lubricant Activity”. American Association of Pharmaceutical Scientists 2004 Annual Meeting Poster Session, the text of which is incorporated herein by reference). In cases where the colorant is loaded onto the outer surface of the talc particles, the amount of colorant load predominantly relates to the surface area and particle size.